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Common maintenance events on the Allison 250 C20
To remove the bronze idler shafts, hold the scavenge cover plate upside down and slap it down over the edge of your work bench. Use your free hand to catch the shafts as they fall out of the cover.
You'll probably see fretting on one side of the brass idler shafts. If it's not too deep, you can salvage the shaft by rotating it 180 degrees. The shafts are installed with Loctite® No. 290. Without the Loctite the shafts can float out of their seats during operation, allowing the slots for the antirotation pins to become exposed in the gear pocket. This can degrade the performance of the pump. In the past, this has been attributed to smoking during and after engine shutdown. Do a dry run before applying the loctite. With the shafts installed in the bottom plate, hold the plate up to the light and look from below to make sure the anti rotation slots on the brass shafts are below the surface. If there is a bit of light peeking through, remove the shafts and sand the ends down a little bit and repeat the dry run.
Common Maintenance Events on the Allison 250 C20
By Jim Taylor
February 1998
Before flushing out the main oil gallery in the gearbox, remove the torquemeter support shaft, the two oil jets, and the return gravity check valve which is located under the filter housing. With the gearbox totally dismantled review the condition of the gearbox cover and housing to determine if you should send them out for rework, or purchase an exchange unit. Things to consider on the cover are:
• Worn 3 & 4 bearing cage, or upgrading to 3 & 4 roller bearings.
• Worn turbine mounting surface known as "surface G."
• Loose or worn bearing cages.
• Incorporation of CEB-A-1226 (through-bolt retention of the idler gears).
• Helicoils® required for the turbine oil scavenge ports.
• Leaking or worn torquemeter insert.
• Bearing bore position suspected due to poor wear pattern on gear teeth.
Items to consider on the gearbox housing are:
• Loose compressor mounting inserts. Note: more than two loose inserts requires the housing be sent to an AMC to have all of the compressor mount inserts checked for being square to the compressor axis.
• Incorporation or CEB-A-1226 (through-bolt retention of the idler gears).
• Worn compressor pilot diameter, causes misalignment of the compressor, resulting in possible high vibrations and an oil leak from the compressor packing.
• Installation of helicoils® in the oil fitting ports.
•Bearing bore position suspected due to poor wear pattern on gear teeth.
The above items should be discussed with your AMC to help you decide what is the best method of accomplishing the repairs.
Assembly tips for the gearbox housing — Keep in mind that these tips are not step-by-step instructions on how to put a gearbox together — you must still refer to the Allison maintenance manual for complete instructions.
